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Full text of "Handbook Of Chemical Engineering - I"

368                              CHEMICAL ENGINEERING
LOSSES IN EVAPORATORS
Radiation.—The outside surfaces of all evaporators, vapor lines, and liquor lines should be covered with an efficient insulating material so as to reduce these losses to a minimum. This is of special importance when evaporators are used intermittently. Average figures have been given in a previous paragraph, " Steam Consumption."
Entrainment is always due to faulty construction or operation of the evaporator. Small particles of liquor are carried to the next steam chest or to the condenser with the vapor, and this can only take place where the vapor speed is too high. Catchalls and entrainment separators will reduce these losses, but evaporators should be designed and operated so that there will be no entrainment.
Foaming or frothing is common with a great many alkaline solutions; and evaporators used for the concentration of such liquors should be especially adapted for. that purpose. In practically all cases, losses by foaming can be prevented by keeping the liquor level as low as possible, in which case the foam bubbles will be broken by coming in contact with the hot surfaces of the tubes that are not covered by liquor. This method of operation will also increase the capacity, as the liquor will be spread in a fine film over a large part of the heating surface. Catchalls and separators are necessary, and will save valuable material in case of careless operation. Uniform working conditions and constant low liquor level are very helpful.
Incrustations.—A great many solutions have the tendency to deposit a scale on that part of the heating surface which comes in contact with these liquors. Such a scale will always cause a considerable loss, not so much in efficiency as in capacity. Actual figures for the reduction in heat transmission have been given in previous paragraphs. Experience has shown that all substances which are less soluble at higher temperatures will form a hard scale on the tubes, while all salts that increase in solubility with rise in temperature will form incrustations that are easily soluble in water; and can therefore readily be removed by boiling. All liquors that have a tendency to deposit a hard scale on the tubes require an evaporator where the tubes can be cleaned mechanically.
Arrangement and Construction of Evaporators.—Local conditions must decide the very important question whether single- or multiple-effect evaporators should be installed. If exhaust steam is to be used only, the number of effects will depend on the quantity available, to which would be added the amount of exhaust steam coming from the pumps of the evaporating equipment proper. In cases where live steam has to be used, it is necessary to compare the saving in fuel with the additional expense for depreciation, interest and repairs of the evaporators. It is natural that for small quantities of liquor, a single or double effect is sufficient, and that only for large plants triple or quadruple effects should be chosen. Of secondary importance is the water consumption for cooling purposes in the condenser, but where water is scarce, it is always advisable to install a cooling tower.
The number of effects is often limited by the maximum temperature at which the liquors can be handled without spoiling them, and in some cases even for large quantities a single effect has to be used, as otherwise the boiling temperature would injure the finished product (gelatin, malt extract). The number of effects is also depend-